The logic in the merger (which mergers all actionproviders found in the lookup) is following:
It iterates all ActionProviders and if the action is among the supported ones and is enabled, it's invoked. If no match is found, the exception is thrown. So the error is either on the side of the caller, which doesn't check if the action is supported and/or enabled or it's on the side of one or more of the implementations that mis-implement the methods.
reassigning to the caller of the ActionProvider

Honza, this looks like web.inspect.ui.CssStylesPanelProviderImpl$1.actionPerformed(CssStylesPanelProviderImpl.java:132) got called on Java Web Project? HTML5 projects does not have LookupProviderSupport$MergedActionProvider in its lookup.

> web.inspect.ui.CssStylesPanelProviderImpl$1.actionPerformed
> (CssStylesPanelProviderImpl.java:132) got called on Java Web Project?
First of all, it is unfortunate that this view is shown for HTML file from Java or Web project. I have filled a separate issue about that - issue 221263. I also updated the view such that it is more robust. It checks for the presence of run.single action and disables the corresponding button when the action is not present. Hence, it is disabled for HTML files in Java and Web projects.
Modified files: http://hg.netbeans.org/web-main/rev/5fab6a20f7cb

(In reply to comment #8)
> Please, fix it for beta2. Css styles view should not be visible for html or
> xhtml files from java or web projects.
It is a bit unfortunate to request this in this issue. Note that this issue is about the IllegalArgumentException only. There is another issue (221263) that is about hiding of Selection section of CSS Styles view in Java/Web projects, but event that issue is not about hiding of the whole CSS Styles view. If you really want that then it would be much better to fill a separate issue.

> Css styles view should not be visible for html or xhtml files
> from java or web projects.
I agree this is a separate discussion (and let's discuss in bug 221463 that Jirka filed), but that part is not a beta stopper. On the other hand, the IllegalArgumentException, plus the fact that selection tab should be hidden if not applicable, ARE beta stoppers. Honzo, we just discussed with Jirka S, can you please transplant any applicable changesets to beta2?
BTW, I see that in trunk the Selection tab is no longer visible when not applicable - can you please post what is the changeset for that?